Such multi-photon processes may involve the simultaneous absorption of two photons. A well-known example is multi-photon fluorescence or higher harmonic generation.
A well-known application is the so-called two-photon excitation microscopy. In two-photon excitation microscopy, a fluorophore is excited by the absorption of two photons simultaneously in one quantum event. Each photon carries approximately half the energy necessary to excite the fluorophore. An excitation then results in the subsequent emission of a fluorescence photon with an energy that is typically higher than that of either of the two excitation photons.
Since the probability of the simultaneous absorption of two photons is very low, high excitation intensities are needed, which in practice are provided by femtosecond lasers which deliver a passively-controlled sequence of very short pulses.
Commercially-available femtosecond lasers are expensive. Also, commercially-available femtosecond lasers are not easy to integrate into other imaging modalities.
In Sebastian Karpf et al., “Time-encoded Raman: Fiber-based, hyperspectral, broadband stimulated Raman microscopy” http://arxivorg/abs/1405.4181, a time-encoded Raman setup using a Fourier Domain Mode-Locked (FDML) laser source together with a dynamically-controllable light source as Raman-pump source is disclosed. The time-encoded Raman setup includes a differential balanced photodetector for detecting a stimulated Raman gain signal. The differential balance photodetector includes InGaAs diodes for receiving near infrared light with a wavelength of more than 1200 nm. This detector is not suitable for detecting signals indicative of a multi-photon process, such as a multi-photon fluorescence signal or a higher harmonics signal.